Most commercially available processes for producing STP produce undesirable fines which cause dusting and difficulties in handling the STP. The fines are typically removed from the larger granules by screening or other methods of separation. The fines are typically not pure STP, and can have up to 20 weight % of other phosphate compounds, such as metaphosphates, pyrophosphates and orthophosphates. The present invention provides a method of utilizing the fines by agglomerating them to produce medium density granular SP.
U.S Pat. No. 3,160,472 discloses the agglomeration of STP fines by calcining the fines in the presence of at least 10% of blends of certain alkali metal orthophosphate salts. U.S. Pat. No. 3,761,573 discloses the spraying of fines with an alkali metal orthophosphate solution to form particles, drying the particles, screening to recover the desired fractions and annealing the recovered particles. U.S. Pat. No. 3,932,590 discloses the spraying of STP fines with a solution of sodium orthophosates and calcining to produce granular STP. U.S. Pat. No. 4,134,963 discloses the agglomeration of STP fines by spraying with water, then calcining. U.S. Pat. No. 4,251,498 discloses the compaction, milling, moisturizing and calcining of STP fines to agglomerate the fines and produce a granular product.
None of the above patents disclose the present invention of agglomerating STP fines with mineral acids to produce medium density SP particles.